Numerous attempts have been made in the prior art to regenerate caustic wash solutions contaminated with dirt accumulations following the cleaning of metal articles. Typically, the contaminants comprise a wide range of materials including particulate matter, oils, greases, inorganic salts, and metal residues which makes regeneration extremely difficult, costly and time consuming.
Because of the difficulty in regenerating caustic wash solutions, and the limited success of prior art attempts, caustic wash solutions have been used to clean metal articles until a heavy dirt load accumulates at which time the wash solutions are discarded.
Filtration techniques have been attempted to regenerate caustic wash solutions, also with limited success. The results of attempts at using filtration techniques have been either the limited removal of contaminants when coarse filters are used or alternatively where fine filters are used the filters are found to plug or require an extremely long period of time to effect filtration. One example of a filtering system for caustic wash solutions is that disclosed in U.S. Pat. No. 3,568,834, which requires filtering cycles of 1 to 5 days.
Another prior art attempt to effect removal of contaminants from wash solutions is that disclosed in U.S. Pat. No. 3,802,917 wherein a small amount of high molecular weight, water-soluble anionic polymer is employed to coalesce tramp oils which then float to the surface from which they are readily separated. One difficulty with the use of these polymers is that the contaminants other than tramp oils remain and build-up in the wash solution. Again, only limited success is achieved in removing contaminants from caustic wash solutions.
Because of the difficulty in removing the contaminants from caustic wash solutions, the prior art has resorted to developing highly involved systems. One such system is that disclosed in U.S. Pat. No. 3,930,879 wherein a portion of the wash water is recirculated to a processing stage where oil and scale are separated continuously. One difficulty which such systems encounter is that they are highly specialized and expensive thereby having limited commercial appeal to processers seeking simple, highly efficient ways to effect removal of contaminants from caustic wash solutions.
Conventionally, in preparing a caustic wash solution, a 50% weight of aqueous caustic solution received from conventional suppliers is diluted to approximately 16 to 18% by weight and is employed to wash metal articles under heat and pressure. During prolonged washing operation, the wash solution is altered in chemical composition corresponding to a gradual increase in contaminants both organic and inorganic in nature.
After prolonged usage of the caustic wash solution in cleaning metal articles, a point of saturation is reached of both organic and inorganic contaminants. Because heat is employed in the cleaning cycle there is a tendency for the wash solution to become supersaturated. Upon cooling the wash solution to ambient temperature, there is a tendency for an increment of contaminants to separate out of the wash solution, the increment corresponding to the temperature differential. This is evident from a layering of the cooled solution with suspended solids and heavier contaminants sinking to the bottom of the vessel and the ligher oils floating on the surface of the wash solution. However, this is not an indication that the caustic wash solution is self-cleaning since the amount of contaminants which separate out as a result of cooling is very small in relation to the total amount of dissolved contaminants in the wash solution.
As the point of saturation is approached for the ontaminants, the wash solution rapidly loses cleaning efficiency such that in order to maintain an adequate degree of cleaning, additional caustic must be added to the wash solution. The additional caustic while increasing the total caustic concentration of the wash solution does not correspondingly increase the cleaning efficiency of the wash solution by any degree which corresponds to the added increment. This is due primarily to the counter effect of the contaminants already present in the wash solution which convert the caustic to a chemical state much less efficient than uncontaminated caustic.
It is also found that after a number of cycles of cooling to ambient temperature following washing of metal articles; layering-removal of contaminants from the wash solution results in less of an increment of contaminants being removed due to supersaturation and repeated additions of make-up caustic. And by addition of make-up caustic, the total caustic strength increases to a point somewhere between about 25 to 30% by weight.
Although the use of cycle treatment and layering removal prolongs the life of the caustic wash solution over an extended period of time, eventually continued addditions of make-up caustic are not justified since they do not correspond to the added expense of the additional dosage. Thus, as the cycles are repeated and the total caustic strength gradually increases, the amount of contaminants removed following cooling gradually decreases. Eventually, a point is reached where the amount of contaminants which are removed is insignificant and the total amount of the caustic wash solution must be replaced.
Certain chemical compounds can be added to the wash solution to increase its efficiency and possibly prolong the useful life. For example, surface active agents may be added which tend to reduce surface tension and hence increase penetrating capacity of the wash solution, while promoting the removal of contaminants by increasing the emulsion properties of the wash solution. These chemical additives are expensive and because they are typically organic compounds, they are subject to being decomposed. Thus, although surface active agents increase the efficiency of the wash solution over a short period of time, their usage in the long run eventually adds to the total amount of organic contaminants in the wash solution.
If the concept of cleaning metal articles is predicated on the eventual disposal of the caustic solution, then there must be some justification for the additional expense of the surface active agents. If, however, the concept of cleaning metal articles is predicated on regeneration of the wash solution, then there is little justification to use surface active agents because of the expense and the fact that they contribute to the contamination problem over the long run.
A number of chemical changes are also found to take place in the wash solution. The caustic which is used to prepare the wash solution contains essentially no sodium carbonate. However, during cleaning, sodium carbonate rapidly builds up in concentration. Although sodium carbonate is sometimes used as an alternative to sodium hydroxide as a cleaning medium, its presence may not be detrimental to the efficiency of the caustic wash solution.
Build-up of sodium carbonate appears to be caused partly by aeration of the wash solution but more likely is caused by the decomposition of organic contaminants under heat and pressure. Decomposition of the organic contaminants releases carbon dioxide which in turn forms the carbonate radicle. This change occurs at the expense of the sodium hydroxide concentration so that almost any sample of the wash solution will show an approximately equal distribution between the sodium hydroxide concentration and that of sodium carbonate.
Inorganic contaminants may account for about 5 to 7% by weight of the total contaminants in a caustic wash solution. These contaminants include phosphate, arsenic, lead, cadmium and other heavy metals which represent a problem in disposal of the contaminated caustic wash solution. Many of these contaminants are actively poisonous and thus require strict disposal requirements.
It has now been found that by practice of the present invention, the difficulties and disadvantages of prior art attempts to remove contaminants from caustic wash solutions have been overcome in a simple, highly efficient manner.